Charged-particle-beam (CPB) microlithography systems (e.g., electron-beam projection-exposure systems) exhibit good imaging resolution but have limits on maximum obtainable throughput (processing speed). Various approaches have been investigated to increase the throughput of these systems. One approach is to increase the area of the pattern portion that can be projection-exposed in one “shot.” Increasing the exposure area raises concerns with beam blur, which poses a need for devices for measuring beam blur and for adjusting the beam so as to reduce blur. For example, blur can be reduced by adjusting one or more of the focal point, astigmatism, magnification, rotation, and other parameters of the beam. Measurements of these parameters can be correlated to overall imaging performance of the system.
Imaging performance as affected by beam blur can be evaluated by either a backscattered-electron (BSE) detection system or a transmitted-electron (TE) detection system. A conventional TE detection system is summarized below and depicted in FIGS. 7 and 8(A)-8(B). Specifically, FIG. 7 is a schematic elevational section, with block diagram, of a conventional device utilizing an electron beam and a knife-edge. FIGS. 8(A)-8(B) are respective plan views of the knife-edged opening used in the device of FIG. 7. FIG. 8(A) depicts the knife-edged opening during beam irradiation, and FIG. 8(B) depicts the knife-edged opening after irradiation-induced contamination has accumulated on the knife-edge.
Referring first to FIG. 7, at the extreme upstream end of the device an illumination-beam source (not shown) is disposed. The illumination-beam source produces an electron beam EB that propagates in a downstream direction to a reticle (not shown) that defines a measurement pattern. The reticle can be the same reticle that defines an actual pattern to be exposed lithographically onto a substrate, or a separate measurement reticle. Downstream of the reticle is an aperture plate 100 defining a single knife-edged reference mark 101, as shown in FIG. 8(A). The electron beam EB is “patterned” by transmission through the upstream measurement pattern on the reticle to have a rectangular transverse profile. The beam EB is incident on the knife-edged reference mark 101 of the aperture plate 100. An electron detector (sensor) 105 is disposed downstream of the knife-edged reference mark 101.
As the rectangular beam EB is scanned in the direction indicated by the arrow in FIG. 7 (to the right in the figure), electrons that strike the non-open portion of the knife-edged reference mark 101 (i.e., electrons that strike the aperture plate 100) are absorbed by the aperture plate 100. Electrons that pass through the opening 102 defined in the aperture plate 100 are detected by the electron detector 105.
The beam current corresponding to the electrons e1 detected by the electron detector 105 is amplified by a preamplifier 106 and converted to a percentage-change-versus-time signal by a differentiating circuit 107. The output wavefront from the differentiating circuit 107 is routed to an oscilloscope 108 or other suitable display. Beam blur is determined from the output waveform, and beam adjustments (e.g., calibration of focal point, astigmatism, magnification, rotation, and various other corrections) are performed as required, based on evaluations of the beam blur so as to improve imaging performance. This method of measuring beam blur is disclosed, for example, in Japan Kôkai (published) Patent Document No. Hei 10-289851 and Japan Patent Application No. 2000-12620.
Unfortunately, the conventional beam-blur-measurement device summarized above is subject to contamination that interferes with proper functioning of the device. As shown in FIG. 8(B), contamination 110 accumulates on the knife-edged reference mark 101 whenever the knife-edge is irradiated for an extended period of time. Contaminant accumulation deteriorates the performance of the knife-edged reference mark 101 and reduces the accuracy of the beam-blur measurements obtained using the device. The contaminant problem can be cured by replacing the knife-edge, but such replacement requires disassembling the device and consequent exposure of the components thereof to air. Having to perform such maintenance on a regular basis substantially reduces the operational efficiency of the microlithography apparatus, and reduces the throughput realized using the apparatus.